Having more time to deliver on a project or think of better ideas or finish a task does not necessarily lead to better results. Many times we can achieve quality output within a short amount of time, creative idea(s) can flow pretty quickly, and you can finish tasks much faster than needed or initially thought it would take to complete.
Setting a deadline to deliver on a project/task can be one of your productivity tools.
If there’s no deadline, you might spend time lingering around than really diving deep into the project/task, lose momentum, lack on motivation or desire or completely forget about it etc.